Pressure difference sensors measure, in accordance with their purpose, measure the pressure difference between a first media pressure and a second media pressure, wherein the range of the pressure difference to be measured is significantly less than the first, respectively the second media pressure. This leads to conflicting goals in the design of the pressure difference sensor, for, on the one hand, it should be able to measure a small pressure difference and, on the other hand, it must withstand high media pressures, wherein, especially in the case of incorrect operation, the pressure difference can indeed reach the order of magnitude of the media pressure. Without protective measures, a measuring membrane of a pressure difference sensor can be destroyed by such a high pressure difference.
Known in the way of protective measures are, for example, overload membranes, which prevent a rise of the pressure difference above a critical value. These overload membranes are integrated in hydraulic measuring aids, which supply the first and second media pressures by means of pressure transfer liquid via hydraulic paths extending, in each case, from a separating diaphragm chamber, which is covered with an isolating diaphragm, to respective sides of the measuring membrane. The overload membrane is connected in parallel with the pressure difference sensor, respectively its measuring membrane. When now the pressure difference exceeds a critical value, the overload membrane is deflected sufficiently far that the entire volume of the pressure transfer liquid in the separating diaphragm chamber on the side of the higher pressure is absorbed by the overload membrane, and the isolating diaphragm comes to rest. Therewith, a further rise of the pressure difference is reliably prevented. The use of overload membranes leads, however, absolutely to greater volume strokes of the pressure transfer liquid and therewith—at equal performance—to greater separating diaphragm areas, which means greater device dimensions and higher costs. Moreover, the measuring mechanism dynamic is influenced negatively by the overload membrane and the greater volume of pressure transfer liquid.
There are, consequently, efforts known, which are directed toward implementing the overload protection for the measuring membrane by membrane beds. In such case, the measuring membrane should upon the exceeding of a limit value for the positive pressure be supported on the membrane bed at least to a degree that the bursting stress of the measuring membrane is not reached even in the case of additional pressure rise.
For such purpose, especially aspherical membrane beds are suitable, which approximate the bend line of the measuring membrane at the limit value for the positive pressure.
The U.S. Pat. No. 4,458,537 discloses a capacitive pressure difference sensor having an aspherical diaphragm bed of glass, which is produced in a structure of coaxial rings, wherein the heights of the rings form a contour, which corresponds to the bend line of the measuring membrane.
Offenlegungsschrift (laid open German application) DE 10 2009 046 229 A1 discloses a pressure sensor, respectively pressure difference sensor, having an aspherical diaphragm bed of glass, which is formed by thermal molding.
U.S. Pat. No. 7,360,431 B2 discloses a pressure sensor, respectively pressure difference sensor, having an aspherical diaphragm bed, which is prepared in silicon by means of grayscale lithography.
Offenlegungsschrift (laid open German application) DE 10 2010 028 773 A1 discloses a pressure sensor, respectively pressure difference sensor, having an aspherical diaphragm bed, which is prepared in silicon by means of laser ablation, followed by an oxidation step and terminal etching.
Japanese Patent JP10078366A discloses a resistive semiconductor pressure sensor having a stepped diaphragm bed, which is prepared in silicon.
U.S. Pat. No. 5,381,299 discloses a capacitive pressure sensor manufactured from silicon with a stepped diaphragm bed, which approximates the bend line of the measuring membrane, wherein the envelope curve of the membrane bed in the central region has a greater radius of curvature then the measuring membrane.
The mentioned pressure sensors have membrane beds, which basically satisfy the mentioned purpose. However, the preparation of the membrane beds, respectively their integration into a pressure sensor, is complex.